Lithographic apparatus and device manufacturing method

ABSTRACT

In an embodiment, a lithographic projection apparatus has an off-axis image field and a concave refractive lens as the final element of the projection system. The concave lens can be cut-away in parts not used optically to prevent bubbles from being trapped under the lens.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as enabling the use of a larger effective NA of the system andalso increasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate (the substrategenerally has a larger surface area than the final element of theprojection system). One way which has been proposed to arrange for thisis disclosed in PCT patent application no. WO 99/49504, herebyincorporated in its entirety by reference. As illustrated in FIGS. 2 and3, liquid is supplied by at least one inlet IN onto the substrate,preferably along the direction of movement of the substrate relative tothe final element, and is removed by at least one outlet OUT afterhaving passed under the projection system. That is, as the substrate isscanned beneath the element in a −X direction, liquid is supplied at the+X side of the element and taken up at the −X side. FIG. 2 shows thearrangement schematically in which liquid is supplied via inlet IN andis taken up on the other side of the element by outlet OUT which isconnected to a low pressure source. In the illustration of FIG. 2 theliquid is supplied along the direction of movement of the substraterelative to the final element, though this does not need to be the case.Various orientations and numbers of in- and out-lets positioned aroundthe final element are possible, one example is illustrated in FIG. 3 inwhich four sets of an inlet with an outlet on either side are providedin a regular pattern around the final element.

SUMMARY

In an immersion lithography apparatus, air (or other gas) bubbles maybecome trapped under a final optical element of the projection systemwhere they may cause one or more faults, e.g. distortion or a blankspot, in the projected image. In particular, if a concave final opticalelement is used, the bubbles will tend to become trapped under the verycenter of the optical element, i.e. on axis, where they could cause mostharm. It can be difficult to arrange a liquid flow that will entrain andremove bubbles from under the center of such an optical element.However, a concave last lens element may be desirable to enable therealize a high NA projection system.

Accordingly, it would be advantageous, for example, to provide animmersion lithographic apparatus having a concave lens element as thefinal element of the projection system in which harmful effects ofbubbles being trapped under the last lens element can be avoided.

According to an aspect of the invention, there is provided alithographic projection apparatus, comprising:

a projection system configured to project a patterned beam of radiationinto an image field that is positioned off the optical axis of theprojection system and onto a target portion of a substrate, theprojection system comprising a concave refractive lens adjacent thesubstrate through which the patterned beam is projected; and

a liquid supply system configured to provide a liquid to a space betweenthe substrate and the lens.

According to a further aspect of the invention, there is provided adevice manufacturing method, comprising:

projecting an image of a pattern through a liquid onto a target portionof a substrate using a projection system, the projection systemcomprising a concave refractive lens adjacent the substrate throughwhich the image is projected and the image field of the projectionsystem being off-axis.

According to a further aspect of the invention, there is provided alithographic apparatus, comprising:

a projection system configured to project a patterned beam of radiationonto a target portion of a substrate, the projection system comprisingan optical element adjacent the substrate and having a surfaceconfigured to be in contact with liquid, the surface being inclined atan angle to a horizontal plane and comprising a first portion throughwhich the patterned beam is configured to pass and a second portion,higher than the first portion, through which the patterned does notpass; and

a liquid supply system configured to provide a liquid to a space betweenthe substrate and the optical element.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts another liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts a further liquid supply system for use in a lithographicprojection apparatus;

FIG. 6 depicts a final lens element and a liquid supply system in alithographic projection apparatus according to an embodiment of thepresent invention; and

FIG. 7 depicts a final lens element and a liquid supply system in alithographic projection apparatus according to another embodiment of thepresent invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam PB (e.g. UV radiation or DUV radiation).    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PL configured to project a pattern imparted to the radiation        beam PB by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables or support structuresmay be used in parallel, or preparatory steps may be carried out on oneor more tables or support structure while one or more other tables orsupport structures are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam PB is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam PB passes through the projection systemPL, which focuses the beam onto a target portion C of the substrate W.With the aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam PB.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamPB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PL. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

Another immersion lithography solution with a localized liquid supplysystem solution which has been proposed is to provide the liquid supplysystem with a liquid confinement structure which extends along at leasta part of a boundary of the space between the final element of theprojection system and the substrate table. Such a solution isillustrated in FIG. 5. The liquid confinement structure is substantiallystationary relative to the projection system in the XY plane thoughthere may be some relative movement in the Z direction (in the directionof the optical axis). See, for example, U.S. patent application Ser. No.10/844,575, hereby incorporated in its entirety by reference. A seal istypically formed between the liquid confinement structure and thesurface of the substrate. In an embodiment, the seal is a contactlessseal such as a gas seal.

FIG. 5 shows a liquid supply system (sometimes referred to as theimmersion hood or showerhead) according to an embodiment of theinvention used to supply liquid to the space between the final elementof the projection system and the substrate. Reservoir 10 forms acontactless seal to the substrate around the image field of theprojection system so that liquid is confined to fill a space between thesubstrate surface and the final element of the projection system. Thereservoir is formed by a liquid confinement structure 12 positionedbelow and surrounding the final element of the projection system PL.Liquid is brought into the space below the projection system and withinthe liquid confinement structure 12. The liquid confinement structure 12extends a little above the final element of the projection system andthe liquid level rises above the final element so that a buffer ofliquid is provided. The liquid confinement structure 12 has an innerperiphery that at the upper end, in an embodiment, closely conforms tothe shape of the projection system or the final element thereof and may,e.g., be round. At the bottom, the inner periphery closely conforms tothe shape of the image field, e.g., rectangular though this need not bethe case.

The liquid is confined in the reservoir by a gas seal 16 between thebottom of the liquid confinement structure 12 and the surface of thesubstrate W. The gas seal is formed by gas, e.g. air or synthetic airbut, in an embodiment, N₂ or another inert gas, provided under pressurevia inlet 15 to the gap between liquid confinement structure 12 andsubstrate and extracted via first outlet 14. The overpressure on the gasinlet 15, vacuum level on the first outlet 14 and geometry of the gapare arranged so that there is a high-velocity gas flow inwards thatconfines the liquid.

FIG. 6 shows a view at substrate level in a lithographic projectionapparatus according to an embodiment of the invention. In theembodiment, the final element of the projection system PL is a concaverefractive lens 30 and the projection system as a whole is arranged toproject an image of the patterning device MA onto an off-axis imagefield EF (also known as the slit). Various known designs of projectionsystem, in particular catadioptric (comprising both reflective andrefractive elements) designs have suitable off-axis image fields. In anembodiment, the final element of the projection system PL may be anoptical element shaped other than a concave refractive lens, such as aplate or curved. Such an optical element has an inclined surface incontact with the liquid, the surface including a portion higher than aportion through which the projection beam PB passes. In this way,bubbles gravitating toward the optical element surface may move up thesurface to the portion of the surface out of the path of the projectionbeam. An example optical element of this type may be a plate having aflat surface in contact with the liquid, a optical element having acurved surface in contact with the liquid, etc.

As the image field EF is off-axis, the projection beam PB is alsolocated off-axis in the final element 30. This means that the finalelement 30 can be cut-away on the side of the optical axis OA away fromthe image field EF. This does not affect the projected image because noradiation would have passed through the cut-away portion. Of course, thelens itself need not be made by making a complete lens and cutting awayparts (though this may be convenient) but can be made directly into thefinal shape, as desired.

The cut-away shape of the final lens element 30 has one or moreadvantages. Firstly, because the lens forms only about half of a dome,there is less, or no, chance of a bubble becoming trapped under thelens—a bubble can easily float through the cut-away. Indeed, because theimage field EF is off-axis, a bubble that remains at the highest pointunder the lens for a little while, will likely be out of the beam pathand hence will not affect the projected image.

Secondly, the cut-away allows provision of a liquid supply 23 above thebottom of the lens 30 so that the immersion liquid flow will begenerally downwards, as indicated by arrows. Such a flow can help ensurethat contaminants that leach out of the resist are kept away from thesurface of the lens.

As shown in FIG. 6, the liquid confinement structure 20 will be shapedto match the shape of the bottom of the projection system but otherwisemay operate on the same principles as known liquid confinementstructures designed for use with projection systems having, for example,circular end parts. For example, the liquid confinement structure 20 maybe provided with an extractor 21 connected to a low pressure source andcovered by a porous membrane to extract immersion liquid and a gas knife22 to prevent leakage of immersion fluid. Also, the holder 31 for thefinal lens element 30 will likely be shaped to fit the cut-away shape.

In a variation of the above described embodiment, shown in FIG. 7, thelens element 30′ retains its rotationally symmetric shape, but isprovided with a through-hole 32 opening at or near the highest point inthe bottom surface of the lens element. The lens element can thereby bemade by conventional rotationally symmetric machining and thethrough-hole provided after, e.g. by drilling or etching. Thethrough-hole 32 is positioned so that it does not interfere with theprojection beam PB and therefore does not affect imaging. Using thethrough-hole 32, the immersion liquid 11 can be refreshed from thehighest point underneath the last lens element 30′ and a liquid flowthat sweeps bubbles away from the center of the lens can be created. Theliquid confinement system can also be made symmetric. If the final lenselement is made of a material that is soluble in the immersion liquid,e.g. CaF₂ in ultra pure water, the through-hole 32 should be lined, e.g.with quartz, to prevent dissolution of the lens element. As discussedabove, the element 30′ may be an optical element shaped other than as alens.

An additional possible advantage of an embodiment of the invention isthat the flow of immersion liquid cools the final lens element at aposition that is close to the projection beam, where it is hottest.Thus, temperature uniformity in the final lens element may improved.

In European Patent Application No. 03257072.3, the idea of a twin ordual stage immersion lithography apparatus is disclosed. Such anapparatus is provided with two tables for supporting a substrate.Leveling measurements are carried out with a table at a first position,without immersion liquid, and exposure is carried out with a table at asecond position, where immersion liquid is present. Alternatively, theapparatus has only one table. In a preferred embodiment, the apparatus,method and/or computer program product as described herein is applied toa single stage/table lithography apparatus.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above and whether the immersion liquid is provided in the formof a bath or only on a localized surface area of the substrate. A liquidsupply system as contemplated herein should be broadly construed. Incertain embodiments, it may be a mechanism or combination of structuresthat provides a liquid to a space between the projection system and thesubstrate and/or substrate table. It may comprise a combination of oneor more structures, one or more liquid inlets, one or more gas inlets,one or more gas outlets, and/or one or more liquid outlets that provideliquid to the space. In an embodiment, a surface of the space may be aportion of the substrate and/or substrate table, or a surface of thespace may completely cover a surface of the substrate and/or substratetable, or the space may envelop the substrate and/or substrate table.The liquid supply system may optionally further include one or moreelements to control the position, quantity, quality, shape, flow rate orany other features of the liquid.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic projection apparatus, comprising: a projection systemconfigured to project a patterned beam of radiation into an image fieldthat is positioned off the optical axis of the projection system andonto a target portion of a substrate, the projection system comprising aconcave refractive lens adjacent the substrate through which thepatterned beam is projected; and a liquid supply system configured toprovide a liquid to a space between the substrate and the lens, wherein,during projection of a beam through the lens and the liquid, the beamdoes not pass through a portion, furthest away from the substrate, of aconcave surface of the lens.
 2. The apparatus according to claim 1,wherein the projection system is a catadioptric system.
 3. The apparatusaccording to claim 1, wherein the lens is cut-away on the opposite sideof the optical axis from the image field.
 4. The apparatus according toclaim 3, wherein the liquid supply system is arranged to provide liquidfrom a position adjacent the cut-away of the lens.
 5. The apparatusaccording to claim 1, wherein the lens is asymmetric about a planecontaining the optical axis.
 6. The apparatus according to claim 5,wherein the liquid supply system is arranged to provide liquid from aposition at or near the optical axis.
 7. The apparatus according toclaim 1, wherein the lens has a first side edge on a first side of theoptical axis and a second side edge on a second side of the opticalaxis, the first side edge being nearer the optical axis than is thesecond side edge and the image field being on the second side of theoptical axis.
 8. The apparatus according to claim 7, wherein the liquidsupply system is arranged to provide liquid from a position near thefirst side edge.
 9. The apparatus according to claim 1, wherein the lensextends substantially only to a position that will be traversed by thebeam.
 10. The apparatus according to claim 1, wherein the lens isprovided with a through-hole opening at or near the highest point of itsconcave side such that liquid can be provided through the through-hole.11. A device manufacturing method, comprising: projecting an image of apattern through a liquid onto a target portion of a substrate using aprojection system, the projection system comprising a concave refractivelens adjacent the substrate through which the image is projected and theimage field of the projection system being off-axis, the image notpassing through a portion, furthest away from the substrate, of aconcave surface of the lens.
 12. The method according to claim 11,wherein the projection system is a catadioptric system.
 13. The methodaccording to claim 11, wherein the lens is cut-away on the opposite sideof the optical axis from the image field.
 14. The method according toclaim 13, comprising providing liquid from a position adjacent thecut-away of the lens.
 15. The method according to claim 11, wherein thelens is asymmetric about a plane containing the optical axis.
 16. Themethod according to claim 15, comprising providing liquid from aposition at or near the optical axis.
 17. The method according to claim11, wherein the lens has a first side edge on a first side of theoptical axis and a second side edge on a second side of the opticalaxis, the first side edge being nearer the optical axis than is thesecond side edge and the image field being on the second side of theoptical axis.
 18. The method according to claim 17, comprising providingliquid from a position near the first side edge.
 19. The methodaccording to claim 11, wherein the lens extends substantially only to aposition that will be traversed by the image.
 20. The method accordingto claim 11, wherein the lens is provided with a through-hole opening ator near the highest point of its concave side and comprising providingliquid through the through-hole.
 21. A lithographic apparatus,comprising: a projection system configured to project a patterned beamof radiation onto a target portion of a substrate, the projection systemcomprising a substantially stationary optical element adjacent thesubstrate and having a surface configured to be in contact with liquid,the surface being inclined at an angle to a horizontal plane andcomprising a first portion through which the patterned beam isconfigured to pass and a second portion, higher than the first portion,through which the patterned beam does not pass; and a liquid supplysystem configured to provide a liquid to a space between the substrateand the optical element.
 22. The apparatus according to claim 21,wherein the projection system is a catadioptric system configured toprovide an image field that is positioned off the optical axis of theprojection system.
 23. The apparatus according to claim 21, wherein theliquid supply system is arranged to provide liquid from a positionadjacent the second portion of the surface of the optical element. 24.The apparatus according to claim 21, wherein the optical element has afirst side edge on a first side of an optical axis of the projectionsystem and a second side edge on a second side of the optical axis, thefirst side edge being nearer the optical axis than is the second sideedge and an image field of the projection system being on the secondside of the optical axis.
 25. The apparatus according to claim 21,wherein the optical element is provided with a through-hole opening ator near the second portion such that liquid can be provided through thethrough-hole.